I have just added a scientific calculator from web2.0calc.com to the resources section of the site. It opens full screen in a separate window when you click on the link. You can leave it open in the background if working on something, and switch windows when you need to use the calculator.
Calculators are very useful and indeed often necessary to allow you to work effectively and efficiently. However, I find that many of the students who come to me for help with maths are over reliant on their calculators. Some to the extent that they are unable to do even very basic mental arithmetic, particularly if it involves fractions. Not only that, they do not see why this is a problem because they think that they can always have a calculator to hand.
I would suggest that students have to do a certain amount of work where calculators are not allowed at various points throughout their course.